1. Field of the Invention
The present invention relates to a zoom lens system and, in particular, relates to a zoom lens system including three or more movable lens groups. The invention also relates to a lens-barrel structure for a zoom lens system including multiple lens groups.
2. Description of Related Art
It has recently become common to use a zoom lens in a phototaking optical system used for a camera. In particular, a camera including a high zoom ratio zoom lens having a zoom ratio of more than three is now typical. Moreover, for a lens shutter type camera and an electric still camera, various zoom lenses suitable for reducing sizes and increasing zoom ratios are proposed, since small-sized lightweight cameras are easily portable.
As such a high zoom ratio lens, a zoom lens configured so that three or more lens groups are moved when the focal length is changed, is typically used. This type of zoom lens is referred to as a multi-group zoom lens. There have been various proposals for multi-group zoom lenses.
In a multi-group zoom lens, there is an increased number of spaces between the lens groups which can be changed when the magnification changes, resulting in a complicated lens-barrel structure. Various proposals related to a lens-barrel structure suitable for a multi-group zoom lens or to a lens-barrel structure suitable for reducing the size of the main body of the camera have been made.
A phototaking optical system has a total lens length that becomes longer as the zoom ratio increases. In order to realize a reduction in size of the main body of the camera, it is necessary to make the phototaking optical system thinner so that it may be accommodated in the main body of the camera when the camera is carried. For example, it is known to provide a two-stage barrel accommodating structure in which the lens barrel is divided into two blocks and is accommodated as if the lens barrel is folded.
However, in order to increase the zoom ratio of a zoom lens in which a few of the lens groups are movable, a change in lateral magnification of each lens group associated with the magnification change becomes large. Therefore, it is essential to control the lens group positions with high accuracy in order to obtain a desired optical performance when manufacturing the zoom lens, and it is difficult to shorten the total lens length of the zoom lens.
Furthermore, in a conventional zoom lens, a ratio of the back focal length to the total lens length is large in the telephoto state. Thus, it is easy for a lens-barrel to fall down in the telephoto state, resulting in tilt of an image of an object. This causes the image of the object to be partially blurred.
Additionally, when the number of movable lens groups is increased, the number of movable portions in the lens-barrel is also increased. This causes a degraded optical performance due to fatigue of the movable portions.